1. Technical Field
The present invention relates to a projection zoom lens system of a projector apparatus for projecting a magnified image displayed on a light valve onto a screen.
2. Description of the Related Art
Conventionally, liquid crystal panels have been commonly used as light valves of projectors. In recent years, in place of the liquid crystal panels, the apparatus having a plurality of elements for forming an image by mechanically changing reflecting direction of light, such as microelectromechanical systems have been brought into practical use. One of examples of such apparatus is a DMD (digital mirror device, deformation micro-foil deviceordisplay) in which fine mirror elements (micro mirrors) are arranged into an array in correspondence with pixels for displaying an image by controlling angle of each of the mirrors. A light modulator for generating the pixels by the micro mirrors provides faster response speed and brighter image compared with the LCD and thus, is suitable for providing a compact projector of high-intensity (contrast ratio) and high-quality image.
When generating an image using the DMD, the micro mirror rotates at about .+-.10.degree. such that effective (on) reflection and ineffective (off) reflection are switched. Therefore, a projector using the DMD as a light valve requires a projector lens system which captures or being into its pupil the effective reflection (effective light or on light) but does not capture the ineffective reflection (ineffective light or off light). Further, in order to minifying the image circle, it is desirable to dispose the projector lens system in the normal direction with respect to the DMD. Therefore, the place for disposing a light source for inputting projection light to the DMD is limited, and in order to input the effective light to the projector lens system from the DMD, it is necessary to dispose the light source in substantially the same direction as the projector lens system.
In the light of above, a projection lens system for a projector apparatus employing the DMD is required to have a small diameter and a long back focus (back focal length) at the light valve side. Generally, in the lens system having a long back focus, the diameter of the lens located rearward tends to become large. However, since the diameter of the rear lens is limited as mentioned above, it is necessary to design the projection lens system such that the position of exit pupil is located in the vicinity of the last (rear) lens.
On the other hand, it is necessary to arrange a large number of lenses so as to enhance the performance of the projection lens system. Disposing a large number of lenses may increase the overall length of the projection lens system to a certain degree. If the overall length of the projection lens system is increased, the diameter of the lens located forward is increased as well in the lens system having the exit pupil located rearward.
A projector apparatus employing the DMD is required to reduce its size and thickness, and the projector lens system is also required to be compact as a whole and to reduce the lens diameter. It is obvious that such a lens system is required to have a sufficiently high aberration performance as the projection lens system. Further, the projection lens system is required to be capable of zooming in addition to satisfy the above-described conditions for the DMD.
It is an object of the present invention to provide a projection lens system, especially a projection zoom lens system suitable for a projector apparatus employing a light modulator, such as DMD, changes reflection directions of light of pixels to form an image. It is an object of the invention to provide a projection zoom lens system having high image-forming performance suitable for the DMD, and to provide a compact projector apparatus employing a light modulator of this type and capable of displaying a bright and high contrast image.